The present invention relates to the operation of a carbon monoxide detector. More specifically, the present invention relates to a method of operating a processor within a carbon monoxide sensor to reduce the calculation complexity required for estimating the COHb percentage.
Currently, natural gas and liquefied petroleum gases are used widely as fuel for domestic, commercial and industrial heating and cooking applications. The wide use of these gases for heating and cooking purposes creates a danger of leakage, which will contaminate the surrounding environment, creating dangerous conditions. In many applications, carbon monoxide is created as a byproduct of the combustion process. Carbon monoxide is an odorless and colorless gas, thus rendering contaminating levels difficult to detect by an individual. Carbon monoxide is absorbed into an individual's lungs and reacts with the hemoglobin in the blood to form carboxyhemoglobin (COHb), which reduces the oxygen carrying capacity of the blood. Thus, the presence of carbon monoxide in an environment above certain levels is extremely dangerous and can easily poison individuals unaware of its presence.
The uptake of carbon monoxide by a human is a function of both the time of exposure to the carbon monoxide as well as the concentration of the carbon monoxide within the atmosphere. The equation for determining the percentage of COHb in a person's blood can be represented by the Coburn, Forster & Kane Equation, which is set forth below.% COHBt=% COHo(e−(t/2398B))+218(1−e−(t/2398B))(0.0003+COppm /1316)  (1)
In the above equation, the variable B is set 0.0404 for representing an individual in a heavy work environment, in which the individual is consuming a relatively large volume of oxygen, while the variable t represents the time in minutes.
A person is in danger of carbon monoxide poisoning when the percentage of COHb in the blood increases due to the continued exposure to carbon monoxide, such as the 10% level set by the Underwriter's Laboratory UL 2034 Carbon Monoxide Exposure Specification. According to the UL 2034 specification, at concentration levels of 70 ppm, a carbon monoxide detector must respond with an alarm within 60-240 minutes. At elevated concentration levels of 400 ppm, the detector must respond with an alarm within an interval of 4-15 minutes.
Although the Coburn, Forster & Kane Equation set forth above is accurate for determining the percent of COHb in a person's blood, the calculation required is very complex and requires a sophisticated and powerful processor for carrying out the complex math operations required to find the solution for the equation. Such complicated math would require extensive math libraries, thus drastically increasing the code size required within the processor operating the carbon monoxide detector. Since many carbon monoxide detectors are battery powered, the increases processor capabilities and additional program memory required to carry out the COHb calculation will result in both higher cost for the product and decreased battery life.
Therefore, a need exists for a method of operating a carbon monoxide detector that reduces the complexity of the COHb calculation while still operating to generate an alarm in accordance with UL standards.